A conventional fuel control system for a gas turbine engine comprises an engine driven main fuel pump and a metering control adapted to sense various engine parameters for controlling the rate of fuel flow to the engine's combustion chambers in accordance therewith. Main fuel pumps and existing fuel control systems are generally fixed displacement gear pumps or vane pumps which provide output flows which exceed engine fuel requirements under certain conditions (e.g., high altitude operations). Such main fuel pumps, therefore, necessitate the bypassing of fuel which engenders fuel heating, impairs pumping efficiency and creates other problems such as detracting from the ability of the fuel to cool engine accessories and oil and hydraulic systems without exceeding a maximum safe engine temperature at the burner nozzles. Moreover, positive displacement pumps require close operating clearances and contain parts with highly stressed metal-to-metal contacts which wear rapidly in low lubricity fuel. In addition, the performance of positive displacement pumps may be adversely affected by the presence of contaminants.
While high speed centrifugal pumps are relatively insensitive to contaminants and are capable of generating the fuel pressure required for engine operation in their normal speed range, under high turndown conditions (low flows at high pump speeds), the fuel in a centrifugal pump becomes unduly heated owing to the dissipation of mechanical energy.
A centrifugal pump arranged to operate with a central hollow core of fuel vapor and a liquid annulus or ring around the pump rotor (such as shown in U.S. Pat. Nos. 3,128,822 and 4,247,263) offers greater efficiency than a conventional centrifugal pump and a consequential reduction in temperature rise in the fluid being pumped. The reason for such reduced temperature rise is that windage losses beget by the impeller become smaller when the radial depth of the liquid annulus is reduced.
Experience with vapor core pumps has shown, however, that for the most critical high turndown ratios (lowest engine flow, which may be only 1% of maximum design flow, at close to maximum speed), a vapor core impeller designed to produce adequate pressure for engine operation at maximum fuel flow, typically imparts more energy to the fuel than is required. The excess energy imparted to the fuel by the impeller is then wasted by inefficiency in the collector and diffuser, which recover static head (pressure) from the fluid dynamic head possessed by the fluid at the point where it leaves the impeller, thereby occasioning temperature rise in the fuel. Also, when vapor core pumps are operated at high speed, low headrise conditions, the outer edges of the impeller blades typically produce cavitation in the fluid ring adjacent the collector. Cavitation bubbles produced in this way often destructively collapse on the collector and diffuser surfaces. The resulting cavitation erosion may cause a significant reduction in pump life.